The present invention relates to the marking of metal for tracking and identification (e.g., information purposes, and more particularly to using laser marked coated metal zones therefor.
Primary metal mills require that their products be accurately identified. Molten metal batches have unique "heat" (batch) chemistries which affect the mechanical properties of the ultimate (further formed) end products. Tracking the many individual pieces produced from a heat is a difficult, time consuming process with many opportunities for error in the stressful (hot, noisy, dimly lit, and physically dangerous) mill environment.
Metals first exit the molten heat as very hot (e.g., 1,800.degree. F. or 982.degree. C.) slabs or billets. Ideally, these slabs and billets should be identified with bar coded information immediately after they solidify and while they are still on the run out tables (before they can be mixed up). Automatic identification (e.g., bar codes) are preferred because they help eliminate the errors inherent in manual marking and reading (estimated by some to be as high as 1 in 300 attempts).
High temperature tags (some with bar codes) have been used for some time. For example, one commercial tag product (supplied by Pannier Corp., Pittsburgh, Pa.) is a relatively thin (e.g. 0.008 in or 0.2032 mm thick) stainless steel tag coated with a high temperature white coating and are printable on-site using a dot matrix impact (inked ribbon) printer. These tags then are manually affixed to the slab or billet using a powder charged or pneumatically driven nail gun. Efforts to automate this prior art tag have generally not been successful because the dot matrix printer mechanism is "delicate" (dot matrix head and ribbon) and does not survive well in the vicinity of hot/dirty products; and the printer ribbon needs frequent replacement (e.g., every 300 tags), especially if high contrast bar codes are desired. Further, the nailing mechanism is difficult to automate as the environment is not conducive to bowl feeders. Nail "sticks" are limited to, say, 50 nails and stick feeds are unproved. Also, nailing becomes less acceptable (it is a foreign imperfection) and attachment is less reliable in premium (harder) grades of metal. Finally, nailing is increasingly unreliable as the product cools (hardens).
Another proposal is found in U.S. Pat. No. 5,422,167 which discloses a label that is formed from a sheet of metal having a face and a back. The sheet face is coated with a layer of coating that is resistant to temperature of the hot metal stock and receptive to being thermally transfer printed. The metal sheet label is of a thickness so that the coating layer can be thermally transfer printed using conventional markers designed for paper or films. The printed label is adapted to be attached to hot (1,200.degree. F. or 648.8.degree. C.) metal stock by welding bare (uncoated) zones of the label. This tag system can withstand the rigors of, for example, steel coil or "hot bands" production and can be attached by welding.
Further, Nierenberg (U.S. Pat. No. 4,323,755) vaporizes a pattern (bar code) on glass CRTs for their identification. To improve contrast, the vaporized area can be coated first. Williams (U.S. Pat. No. 5,206,280) discloses a laser markable white pigment composition. Shimokawa (U.S. Pat. No. 4,847,181) proposes a dual layer label that can be laser marked. Gnanamuthu (U.S. Pat. No. 4,716,270) proposes a laser marking system where substrate is etched following laser marking of a label. Norris (U.S. Pat. No. 5,262,613) retrofits a mechanical engraver with a laser. Snakenborg (U.S. Pat. No. 4,946,763) proposes form a pattern in a metal stencil which is covered by a resist material containing a high concentration of metal powder. Resist material is removed by a laser beam to form the pattern. Honaker (U.S. Pat. No. 4,935,288) proposes a laser printable label having a coating of laser printable acrylic. Kiyonari (U.S. Pat. No. 5,063,137) proposes a resin composition for laser marking having an inorganic compound, like an anhydrous metal borate salt, and a resin. Kiyonari (U.S. Pat. No. 5,035,983) proposes a laser marking composition containing a non-black inorganic lead compound. Azuma (U.S. Pat. No. 4,861,620) proposes a pigment layer which can be marked by a laser beam. Herren (U.S. Pat. No. 5,030,551) laser marks ceramic materials coated with a transparent layer of titanium dioxide. Gernier (U.S. Pat. No. 4,985,780) proposes a two carriage assembly for laser marking articles.
Still, there exists a need in the art for a tag and identification system that can withstand the rigors of primary metal mills and in which the tag production and affixation are automated in order to provide significant labor savings (e.g., at least 1 worker per shift) and to eliminate the errors resulting from manual application (e.g., shuffled tags, sequences out of step by one, etc.).
Additionally, a variety of other raw and finished goods (e.g., automobile mechanical parts, tires, etc.) require marking for identification or information purposes. Such goods may be at or below room temperature when the marking requirement arises. A system that has the flexibility to mark "hot" metal as well as lower temperature items would be welcome.